Retroreflective materials have the property of redirecting light incident on the material back towards its originating source. This advantageous property has led to the wide-spread use of retroreflective sheeting on a variety of articles. Very often the retroreflective sheeting are used on flat inflexible articles, for example, road signs and barricades; however, situations frequently arise which require the sheeting to be used on irregular or flexible surfaces. For example, a retroreflective sheeting may be adhered to the side of a truck trailer, which requires the sheeting to pass over corrugations and protruding rivets, or the sheeting may be adhered to a flexible substrate such as a road worker's safety vest or other such safety garment. In situations where the underlying surface is irregular or flexible, the retroreflective sheeting desirably possesses the ability to conform to the underlying substrate without sacrificing retroreflective performance.
There are two common types of retroreflective sheeting: microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as "beaded" sheeting, is well known in the art and employs a multitude of microspheres, typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Illustrative examples of such retroreflectors are disclosed in U.S. Pat. No. 3,190,178 (McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No. 5,066,098 (Kult).
Basic cube corner retroreflective sheeting is well-known to those of ordinary skill in the retroreflective arts. The sheeting comprises a substantially planar base surface and a structured surface comprising a plurality of cube comer elements opposite the base surface. Each cube-corner element comprises three mutually substantially perpendicular optical faces that intersect at a single reference point, or apex. Light incident on the planar base surface of the sheeting is refracted at the base surface of the sheeting, transmitted through the sheeting, reflected from each of the of the three perpendicular cube-comer optical faces, and redirected toward the light source. The symmetry axis, also called the optical axis, is the axis that extends through the cube corner apex and forms an equal angle with the three optical surfaces of the cube corner element. Cube corner elements typically exhibit the highest optical efficiency in response to light incident on the base of the element roughly along the optical axis. The amount of light retroreflected by a cube corner retroreflector drops as the incidence angle deviates significantly from the optical axis.
Manufacturers of retroreflective sheeting design retroreflective sheeting to exhibit its peak performance in response to light incident on the sheeting at a specific angle of incidence. The term `entrance angle` is used to denote the angle of incidence, measured from an axis normal to the base surface of the sheeting, of light incident on the sheeting. See, e.g. ASTM Designation: E 808-93b, Standard Practice for Describing Retroreflection. Retroreflective sheeting for signing applications is typically designed to exhibit its optimal optical efficiency at relatively low entrance angles (e.g. approximately normal to the base surface of the sheeting). See, e.g. U.S. Pat. No. 4,588,258 to Hoopman. Other applications such as, for example, pavement marking or barrier marking applications, require retroreflective sheeting designed to exhibit its maximum optical efficiency at relatively high entrance angles. For example, U.S. Pat. No. 4,349,598 to White ('598 patent) discloses a retroreflective sheeting design wherein the cube corner elements comprise two mutually perpendicular rectangular faces disposed at 45 degrees to the cube comer sheeting base and two parallel triangular faces perpendicular to the rectangular faces to form two optically opposing cube corner elements. U.S. Pat. No. 4,895,428 to Nelson, et al. ('428 patent) and U.S. Pat. No. 4,938,563 to Nelson, et al. ('563 patent) disclose a retroreflective sheeting wherein the cube corner elements comprise two nearly perpendicular tetragonal faces and a triangular face nearly perpendicular to the tetragonal faces to form a cube corner. The cube corner elements further include a non-perpendicular triangular face.
The manufacture of retroreflective cube corner element arrays is typically accomplished using molds made by different techniques, including those the techniques known as pin bundling and direct machining. Molds manufactured using pin bundling are made by assembling together individual pins which each have an end portion shaped with features of a cube corner retroreflective element. U.S. Pat. No. 3,632,695 (Howell) and U.S. Pat. No. 3,926,402 (Heenan et al.) disclose illustrative examples of pin bundling. The direct machining technique, also known generally as ruling, comprises cutting away portions of a substrate to create a pattern of grooves that intersect to form structures including cube corner elements. The grooved substrate is typically used as a master mold from which a series of impressions, i.e., replicas, may be formed. In some instances, the master itself may be useful as a retroreflective article. More commonly, however retroreflective sheeting or retroreflective articles are formed in a polymeric substrate using the master mold or using replicas of the master mold.
Direct machining techniques are a useful method for manufacturing master molds for small microcube arrays. Small microcube arrays are particularly beneficial for producing thin retroreflective sheeting that has improved flexibility. Microcube arrays are also more conducive to continuous manufacturing processes. The process of manufacturing large arrays of cube corners is also relatively easier using direct machining methods rather than pin bundling or other techniques. An illustrative example of direct machining is disclosed in U.S. Pat. No. 4,588,258 (Hoopman).
Master molds suitable for use in forming cube corner sheeting in accordance with the '598 patent, the '428 patent, and the '563 patent may be formed using direct machining techniques as described above. However, the cube corner geometries disclosed in the these patents require two different machining tools to produce a master mold. This reduces the efficiency of the master mold manufacturing process. Additionally, master molds manufactured according to these patents comprise surfaces that extend substantially perpendicular to the base surface of the master mold. Such perpendicular surfaces can be detrimental to the process of producing exact replicas of the master mold.